THIS ADVISORY HAS EXPIRED
Issued: March 19, 2014 at 7:00 a.m.
Valid Until: 11:59 p.m. of issue date
Good morning! This is Erich Peitzsch with the Flathead Avalanche Center avalanche advisory for Wednesday, March 19, 2014. This advisory applies only to backcountry areas outside established ski area boundaries. The next scheduled advisory will be Saturday, March 22, 2014.
The most recent storm dropped up to 10-18 inches within our advisory area and tapered late Monday night (SNOTEL and remote weather stations). Winds gusted into the 25-30 mph range. Partly to mostly cloudy skies dominated yesterday with moderate wind. Currently, mountain temperatures range from 17-25 °F with winds out of the southwest at 5-30 mph with gusts to 45 mph in southern Glacier Park. No new snow fell overnight, but another disturbance rolls through the area this afternoon through tomorrow. Today, temperatures should range from the mid-20s to low 30s F with winds 10-20 mph and gusts to 30 mph. Snow should begin this afternoon and we could see up to 6 inches by tomorrow morning at upper elevations within the advisory area.
On Tuesday we traveled in the Middle Fork corridor of the northern Flathead Range. We found up to 12-18 inches (30-45 cm) of new snow. On leeward slopes we found sensitive wind slabs (photo). These were easily triggered and up to a foot thick. As they traveled downhill they would entrain both dry and wet (depending on aspect and elevation) sluffs that ran several hundred vertical feet. These sluffs were also fairly easy to trigger on steep, small test slopes (photo).
We also found the early March rain crust about 30 inches (80 cm) from the surface (profile). This crust is about 8 inches (20 cm) thick and harbors softer moist, weak snow below it (video). Most sun exposed slopes formed yet another sun crust on the surface by late in the day. The new snow on these slopes became wet and heavy throughout the day.
BNSF Avalanche Safety reported three small soft slab avalanches in John F. Stevens Canyon in southern Glacier Park yesterday involving the new storm snow. These occurred on high elevation east facing slopes and probably during the storm on Monday (photos). These slabs ran about 100-200 vertical feet. They also reported 12 inches (30 cm) in the canyon floor and 18 inches (45 cm) at upper elevations.
Skiers in the Essex area in the Flathead Range on Monday reported a few natural sluffs within the new snow and localized cracking while skiing during the storm. They found various layers to be somewhat reactive with moderate to hard force in their stability tests including the layer of moist, weak snow below the thick early March rain crust.
We received no observations from the Swan Range or the Whitefish Range so our confidence in these areas is fair.
Avalanche Problem #1
Wind slabs were easy to trigger yesterday, and should be again today. Increasing winds and a bit of new snow this afternoon will continue to form wind slabs. These slabs weren’t particularly wide, but even a slab up to a foot deep can have high consequences. The slabs observed by BNSF Avalanche Safety occurred on easterly aspects which were loaded by wind during the recent storm. Look for and avoid convex pillows of wind-drifted snow on the lee side of ridges and other terrain features. Wind slabs are confined to leeward slopes and cross-loaded terrain features and can be avoided by sticking to sheltered or wind-scoured areas.
Avalanche Problem #2
Several layers within our snowpack pose a persistent slab and deep persistent slab threat. The late January/early February crust/facet layer that was the weak layer responsible for the large, natural avalanche cycle almost two weeks ago still lurks. Even though it is buried deeply the possibility for avalanches to fail or step down on this layer still exists, particularly in areas of shallow snow as demonstrated by BNSF Avalanche Safety less than a week ago (video). The rain event that provided the trigger for that large cycle formed a thick rain crust that now sits about 2-3 feet from the surface. Weak, moist snow sits below this crust and is also a concern. While we have yet to hear or observe any reactivity of this layer it cannot be ruled out as a potential persistent slab problem. These layers may be harder to trigger now, but the consequences of an avalanche breaking on these layers are high. Given the uncertainty of these deep slabs, it is best to avoid slopes where these layers exist or just avoid steep slopes altogether.
Also, cornices are ripe and ready to fail especially during times of warm weather or rain. They can trigger deeper layers when they hit the slope so stay off of and out from underneath these giant monsters.
Oh, Spring in northwest Montana! Spring can bring a mixed bag of weather and snow conditions. It is not uncommon to have sun, snow, and rain in the mountains all in one day during this time of year. It is important to pay attention to rapidly changing conditions as weather can greatly affect snow and avalanche conditions. Pay particular attention to sun exposed slopes as the day progresses as well as rain on snow as wet avalanche conditions can change quickly.
For today, the avalanche hazard is CONSIDERABLE on wind loaded slopes and MODERATE on all other slopes This means that human triggered avalanches are likely on slopes where wind formed sensitive wind slabs. Deeper instabilities within the snowpack exist and it is still possible to trigger an avalanche on these deeper layers particularly in areas of shallow snow and steep slopes. It is also possible to trigger an avalanche involving the recent storm snow. Cautious routefinding and conservative decision making are essential. Identify areas of wind drifted snow and weak layers buried deeper in the snowpack and avoid these areas.
Note: The accuracy of the avalanche advisory becomes much more robust when we have more information. Thus, observations from all of you are extremely valuable to us. Even it is just a simple email saying “Hey, we found good riding in Mountain Range X, and observed no signs of instability or recent avalanches”. This type of information is just as important as observations of avalanches. The observations need not be formal, and can remain anonymous. Don’t’ worry, we won’t give away your secret riding/skiing spot either. Call us at 406.261.9873 or email us at firstname.lastname@example.org. Thanks for your help.
See recent snow profiles as well as snow profiles from the entire season here.
Check out an interesting new research project that you can participate in about winter backcountry riding/snowmobiling and decision making from the Snow and Avalanche Lab at MSU. Details here.
This advisory applies only to backcountry areas outside established ski area boundaries. This advisory describes general avalanche conditions and local variations always occur. This advisory expires 24 hours after the posted time unless otherwise noted. The information in this advisory is provided by the USDA Forest Service who is solely responsible for its content.